bossy person; self-appointed manager; bossyboots
Colloquial, mildly derogatory term for someone who takes charge without being asked, often organizing or directing others in a pushy way.
He's always a bossyboots, wanting to decide everyone's plans.
That self-appointed manager is running the meeting again.
Compound of 仕切り (managing, partitioning) from verb 仕切る and suffix 屋 (person with a trait). Literally 'a person who manages/partitions', used for someone who bossily takes charge.